[cairo-bugs] [Bug 84330] record-paint-alpha-clip-mask.xcb.rgb24 random failure

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=84330

--- Comment #3 from Uli Schlachter <psychon at znc.in> ---
Sorry that I missed this report for two years. No idea how that happened.

Also: sorry, but I think that this might be a bug in the X11 server.
_composite_traps() is a wrapper around
_cairo_xcb_connection_render_trapezoids() that converts from "cairo" to "X11",
so this doesn't do anything special here.

So, we are sending a Trapezoids-request with an empty list of traps.

Looking at the render spec, a Trapezoids-request with an empty list of traps
should just copy the source to the target. I bet some code is incorrectly
optimizing this away, assuming it is a no-op (which it would e.g. be for
operator OVER or ADD).

And indeed, the bug is easy to find:

https://cgit.freedesktop.org/xorg/xserver/tree/render/render.c?id=e8e36755abb17872d669b88d33ca9adc511029a0#n745

Does anyone have the time to test if this bug is fixed if the above "if
(ntraps)" is just removed?

(Afterwards, I guess we should still work around this in cairo, but at least we
will also get the Xorg bug fixed and we can have a comment saying why this is
needed.)

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